1. Field of the Invention
The present invention relates to a Time Division Duplexing (TDD)-based wireless communication system, and in particular, to a method of establishing a TDD time frame.
2. Description of the Related Art
In a mobile communication system repeaters are used to extend cell coverage of the system. In particular, in the case of a basement or the inside of a building in which electronic wave propagation is limited or non-existent, optical repeaters using an optical link are widely used. The optical link is used to transmit a wireless signal through an optical repeater and also used to transmit a wireless signal to an antenna far from a base station. The optical repeaters are configured and used for transmitting a wireless signal of Code Division Multiple Access (CDMA) or Wideband CDMA (WCDMA) corresponding to current mobile communication systems.
Mobile communication systems use a Frequency Division Duplexing (FDD) method using different frequencies and a Time Division Duplexing (TDD) method using different times as a duplexing method for distinguishing an uplink (UL) from a downlink (DL) for transmission of a wireless signal.
CDMA and WCDMA mainly use the FDD method, and the TDD method is discussed for Wireless Broadband Internet (WiBro) and 4th Generation Mobile Communication (4G), which correspond to new type mobile communication systems.
According to characteristics of the TDD method, as Uplink (UL) and Downlink (DL) data amounts vary and a channel characteristic of a UL Radio Frequency (RF) signal is the same as that of a DL RF signal, it is considered that the TDD method is suitable for next generation mobile communication systems in which multiple antennas are used. In addition, the TDD method is different from the FDD method in that UL data is transmitted at a predetermined time while DL data is transmitted at the other time.
FIG. 1 is a schematic configuration of a conventional TDD system in which a time delay occurs. In FIG. 1, it is assumed that a wireless base station system time delay occurs as a Base Station (BS) is separated from a Relay Station (RS). The BS and the RS may be connected to each other in a wireless manner or through a wired transmission medium such as an optical fiber. In the case where the BS and the RS are connected to each other in a wired manner, a time delay between the BS and the RS may be referred to as tfiber and the time delay of the BS in processing the data may be referred to as tbs. Thus, time taken for data generated by the BS be emitted from an antenna of the RS may be referred to as t_fix, and represented as t_fix=tfiber+tbs.
When the wireless base station system transmits wireless data in the TDD method, a signaling diagram is illustrated in FIG. 2.
FIG. 2 illustrates a typical signaling diagram of UL and DL signals in a BS and an RS in a time axis. In FIG. 2, a duration time of a DL is defined as t_DL, and DL data having the duration time of t_DL is emitted from an antenna of the RS after time t_fix after generation in the BS. Assuming the time delay between the RS and a Mobile Station (MS) is negligible (i.e., a distance between the RS and the MS is short enough to neglect the time delay between the RS and the MS), the MS receives the DL data a time t_fix after the DL data is generated by the BS.
As the MS transmits UL data after receiving the DL data, the MS transmits UL1 data after receiving DL1 data as illustrated in FIG. 2. As illustrated in FIG. 2 the MS has a Transmit/receive Transition Cap (TTG), that represents a time delay from reception of DL data to transmission of corresponding UL data. UL data generated by the MS arrives at the BS after the time delay of t_fix.
Thus, in this wireless BS system, the BS receives UL data a time (t_fix+t_fix+t_DL+TTG) after transmitting DL data.
In the typical wireless BS system, if tfiber or t_fix is large, because the distance between the RS and the BS is large, a time delay taken from transmission of DL data to reception of UL data is significantly large, a decrease of capacity of the wireless BS system is noticed. When efficiency of a TDD wireless system is defined as a ratio of the sum of a DL data duration time and a UL data duration time to a total time, the efficiency can be represented by Equation 1.Efficiency=(t—UL+t—DL)/(t_fix+t_fix+t—DL+t—UL+TTG)  (1)
In order to increase the efficiency of a TDD wireless system, which is represented by Equation 1, the time of t_UL or t_DL must be very long, or the time of t_fix must be very short. However, since t_UL is not sufficiently long compared to t_fix in many wireless BS systems, TDD wireless system efficiency decreases due to a time delay of each of a BS and an RS.